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Random walk on a 100-point circle

Question

Points 0,1,…,990,1,\dots,99 are arranged in order around a circle. A walker starts at point 11 and at every step moves to one of its two neighbours, each with probability 12\tfrac12. What is the expected number of steps until the walker first reaches point 00?

Short answer

9999. Cut the circle at 00: the walker sits 11 step from 00 one way and 9999 steps the other way. For a symmetric simple walk between barriers at distances aa and bb, the expected exit time is abab, here 1×991\times 99.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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